Xerographic reproduction apparatus use a photoreceptor in the form of a drum or a belt in the creation of electrostatic images upon which toner is deposited and then transferred to another electrostatically charged belt or drum, or to paper or other media. Once the toner image is transferred, most xerographic apparatus clean the photoreceptor in ways that can abrade the surface, changing the thickness of the photoreceptor over time. Even without such abrasion, the thickness of the photoreceptor will decrease through use over time. Because of the nature of the photoreceptor, a change in thickness will result in a change in its electrostatic performance, which can be measured by the “dielectric thickness” of the photoreceptor. To ensure consistent output from xerographic apparatus, an assessment of the state of the photoreceptor is very useful.
In addition to the dielectric thickness, the thickness and surface potential of a photoreceptor can be used to assess its state. Thus, measurements of the photoreceptor thickness and surface potential can be used to evaluate and/or stabilize performance in a xerographic marking engine. Robust and more consistent performance can be achieved by varying xerographic control factors based on these measurements. Surface potential and thickness can be measured using electrostatic voltmeters (ESVs) and actual thickness sensors. However, ESVs would be costly to implement, particularly in color xerographic apparatus including multiple photoreceptors and/or marking engines. Instead, such xerographic apparatus typically estimate the condition and thickness of the photoreceptor indirectly by tracking the photoreceptor cycle count and assuming that the photoreceptor wears at a constant rate as a function of cycle count. This assumption tends to be inaccurate, leading to inconsistent performance over the life of a photoreceptor and potentially premature disposal of the photoreceptor. Thus, there is a need for an accurate method of measuring the thickness and/or surface potential of a photoreceptor without using electrostatic voltmeters, actual thickness sensors, or assumptions of wear rate as a function of photoreceptor cycles.
U.S. Pat. No. 6,611,665 to DiRubio et al., incorporated by reference above, discloses a method and apparatus using a biased transfer roll as a dynamic electrostatic voltmeter for system diagnostics and closed loop process controls. While the techniques disclosed in the '665 patent are useful, they can suffer inaccuracies due to unpredictable aging effects of the elastomers used in the BTR, as well as other factors.
Embodiments provide much more accurate measurements by using the biased charging roller to measure both the photoreceptor surface potential (VOPC) and the photoreceptor dielectric thickness (DOPC). Other current marking engines employ costly Electrostatic Voltmeters (ESVs) to measure the photoreceptor surface potential (VOPC) to measure surface potential. In the case of tandem marking engines, which use four photoreceptors as seen, for example, in FIG. 1, at least four ESVs would be required, which increases the cost of the marking engine significantly. Thus, embodiments, by using existing subsystem components to measure photoreceptor surface potential with only minor modifications to the power supply, allow measurement, control, and adjustment with little increased cost.
The measurement routine of embodiments can be run periodically, such as during cycle-up or cycle-down, to ensure consistent output of the xerographic apparatus in which it is used. VOPC is measured in embodiments by operating the biased charging roller in a constant DC current mode and measuring the DC voltage applied to the shaft by the power supply, which will shift in response to VOPC. DOPC is measured in embodiments by first charging the photoreceptor with the biased charging roller operated in a DC biased AC mode, then measuring VOPC with the biased charging roller. Preferably, the charging and measuring is repeated for multiple values of AC biased charging roller peak-to-peak voltage (VP-P) above and below the bipolar VP-P charging knee. The location of the knee, which is a measure of DOPC, can then be calculated. Xerographic process stability is achieved by subsequently adjusting ROS, charging, development, erase, transfer, and other xerographic control factors based on the results of the measurements of DOPC and VOPC.
Employing embodiments to directly measure photoreceptor surface potential VOPC using existing hardware in the engine thus enables more advanced process controls and machine self-diagnoses, yet does not significantly increase manufacturing costs and requires only minor modifications to the biased charging roller power supply to add this functionality. The performance of any subsystem that impacts the photoreceptor charge (erase, pre-transfer, transfer, discharge, development etc.) can be evaluated and/or adjusted using subsystem actuators. Likewise, the performance of any subsystem that is impacted by the photoreceptor charge, such as erase, pre-transfer, transfer, discharge, development, and other components, can be evaluated and/or adjusted using subsystem actuators. Additionally, subsystem failures can be detected, allowing the controller to generate an error message or initiate a service call through remote diagnostics. Additionally, automated Photo-Induced Discharge Curves can be generated using embodiments.
Embodiments enable direct measurement of the photoreceptor dielectric thickness, DOPC, and therefore the photoreceptor thickness, using existing hardware in the engine. Since many xerographic machines currently use a prediction equation that is based on the number of photoreceptor cycles to estimate OPC dielectric thickness, employing embodiments provides much more accurate thickness determination, which allows more advanced process controls and machine self-diagnoses. Thus, marking system performance can be optimized by adjusting subsystem actuators (development, charge, discharge, transfer, erase, etc.) based on DOPC. Further, because photoreceptor/CRUs are currently replaced after a fixed number of cycles, the more accurate measure of DOPC enables a better estimate of photoreceptor age and performance, reducing run cost by potentially reducing the frequency at which the unit is replaced. Other benefits of employing embodiments include improved marking stability and image consistency. Embodiments can be employed cheaply by any engine that uses BCRs. BCRs are widely used in color and black and white office products by all major manufacturers of xerographic engines.